Spray powders are used to produce coatings on substrates by means of “thermal spraying”. In this process, pulverulent particles are injected into a combustion flame or plasma flame which is directed onto a (usually metallic) substrate which is to be coated. The particles here melt completely or partially in the flame, impinge on the substrate, solidify there, and form the coating in the form of solidified “splats”. In contrast, in cold gas spraying, the particles melt only on impingement on the substrate to be coated as a result of the kinetic energy set free. It is possible to produce coatings having a layer thickness of several μm up to several mm by thermal spraying.
A frequent application of spray powders is the production of wear protection layers. These comprise, both in the case of the layers as well as in the case of the powders, typically cermet powders, which are distinguished in that they firstly contain a hard material (this is the ceramic component, “cer-”), most frequently carbides such as tungsten carbide, chromium carbide, and more rarely, other carbides, and secondly, a metallic component as metallic matrix (“-met”) which consists of metals such as cobalt, nickel and alloys thereof with chromium, more rarely also iron-comprising alloys. Such spray powders and sprayed layers produced therefrom are thus classic composites. Such spray powders are also known to those skilled in the art as “agglomerated/sintered” spray powders, i.e., agglomeration (also referred to as pelletization) is firstly carried out in the production process and the agglomerate is then internally thermally sintered in itself in order to give the agglomerates the mechanical stability necessary for thermal spraying. However, those spray powders which are produced by sintering of powder mixtures or pressed bodies followed by a comminution step also meet the necessary prerequisites. These types of spray powders are known to those skilled in the art as “sintered/crushed”. The two abovementioned types of spray powders are, for example, typified by the standard DIN EN 1274:2005. Both classes of powder are also described as “sintered spray powders”.
Sintered/crushed spray powders are produced in a manner analogous to agglomerated/sintered powders with the exception that the powder components must not necessarily be mixed wet in dispersion, but can be mixed dry and optionally tableted or compacted to provide shaped bodies. The subsequent sintering is carried out analogously, however, compact, solid sintered bodies are obtained which must then be converted back into powder form by mechanical force. The powders thereby obtained have an irregular shape and are characterized by fracture processes on the surface. These spray powders have significantly poorer flowability, which is disadvantageous for a constant deposition efficiency (deposition rate) in thermal spraying.
Coatings can be characterized by empirically determinable materials properties in a manner analogous to solid materials. These include hardness (for example, Vickers, Brinell, Rockwell, and Knoop hardness), wear resistance (for example, in accordance with ASTM G65), cavitation resistance, and friction behavior, but also the corrosion behavior in various media and density, in particular true density. In the case of coatings formed by cermets, the materials properties are determined by the proportion and degree of distribution of the metallic phase and the ceramic or hard material phase. The fundamental relationships therefor are familiar to those skilled in the art. One of these relationships is the Hall-Petch law. This law establishes the connection between the degree of dispersion of the ceramic phase and various materials properties. It follows that the ceramic or hard phase should be dispersed as finely as possible in the metallic phase if high strength and high hardness are to be achieved. For this purpose, the metallic phase must have a preferably complete contiguity. This means that it forms a complete three-dimensional network in the mesh gaps of which the hard material particles are embedded and thus separated from one another.
For some applications, a low true density of coatings with cermets, particularly in the case of moving, in particular rotating and/or flying components, can be advantageous. The geometric density of a coating is here close to the true density, which is calculated from the volume-weighted proportions of the components (e.g., the hard materials, the metallic matrix, and potential oxidation products), and their true densities. The true density can, for example, be determined on full-density coatings after detachment thereof via of the Archimedes method. The true density of pulverulent coating materials can be determined as pure density, for example, as skeleton density, via pycnometry, in particular via helium pycnometry (DIN 66137), with the measured values being very close to the true density in the case of “completely” open-pore powders. Under ideal conditions, the value for the true density of single-phase powders or bodies is identical to the density determined by the X-ray method.
To obtain the necessary polishability of coatings in order to achieve very low roughnesses, as is necessary in the case of tribologically stressed layers, the hard materials present in the coating must have a sufficiently good distribution in the metallic matrix and have a small size. It follows therefrom that the metallic matrix should also have a web width (ridge width) which is of the same order of magnitude as is likewise necessary for polishability. A low web width of the metallic matrix leads, in the case of cermet powders, to a low elongation at break, which improves polishability.
The web width of the metallic matrix is defined as the average distance between neighboring hard material particles in the coating which is filled with the metallic matrix. The greater this web width, the greater the maximum absolute elongation at break, and the greater the deformed regions and thus also the roughness in the polishing operation.
It is clear therefrom why thermal spraying of powder mixtures (known as “blends”) is not advantageous: the powders used must have a certain minimum size, i.e., because of the turbulences in the flame, which typically lies in an average particle size range of from 15 to 100 μm. This results, however, in the coating having a heterogeneous texture (“spot landscape”) made up of the powder types used. The consequence is that matrix and hard material are not dispersed in the μm scale, with adverse consequences for polishability. Typical examples of a blend of agglomerated/sintered Mo/Mo2C with an alloy powder may be found in the patent EP 0 701 005 B1. Coatings having a lamellar microstructure result from the use of NiCrFeBSi alloy powder as a metallic matrix, which does not contain any hard materials, and therefore produces the hard material-free, metallic lamellae described. The material's advantages which would result from a high degree of dispersion of the metallic phase in the hard material therefore cannot be achieved by a blend.
The chemical state of the surface is important for the mixed friction region according to Stribeck. Soft oxides as surface species, which can be detected, for example, by surface-analytical methods, are advantageous. These are advantageously soft layer lattice oxides such as B2O3, WO3, or MoO3, and the hydration acids thereof. These have, for example, a strong, positive influence on the break-off moment after long inactivity of the friction pairing, as can occur, in particular, in the case of hydraulic piston rods or else in the case of piston rings.
A coating used in the prior art is electrochemically produced hard chromium. A disadvantage thereof is the strongly environment-polluting production from hexavalent chromium, which is classified as a carcinogenic. An advantage is the very low coefficient of friction (μ). Additional disadvantages are tensile stresses and cracks resulting therefrom which do not produce effective corrosion protection of the substrate. The coating which is under tensile stress also represents a weakening of the substrate in respect of its mechanical cycling strength (fatigue). The cracks also sometimes transport hydraulic oil containing toxic constituents such as ethyleneamine into the environment when a piston rod is taken out. Hard chromium has virtually no elongation at break and is therefore readily polishable (to an average peak-to-valley height (scallop) of 0.1 μm), but is brittle in the case of mechanical shock. The wear resistance tends to be moderate because of a lack of hard materials. The geometric density is comparatively low at about 7 g/cm3. It is thus below the true density of metallic chromium (7.19 g/cm3). The cause therefor is pores and cracks.
Fusible materials based on Ni or Co—CrFeBSi (for compositions, see, for example, DIN EN 1274:2005, Table 2) display extraordinarily dense, i.e., relatively nonporous, layers. After melting of the initially porous sprayed layer, very hard but also very brittle CrB precipitates are obtained. Fusible materials display a very low coefficient of friction, presumably due to the boron trioxide present on the surface which is known to have good properties as a solid lubricant. The fusible materials also display very good polishing behavior, but have little wear resistance because of the very low elongation at break (similar to the case of hard chromium). They are therefore often processed in an admixture (as a blend) with other hard material-containing spray powders, e.g., with WCCo 88/12 or 83/17, or else with metallic molybdenum which often contains Mo2C precipitates, or even with pure molybdenum carbide spray powders. The latter coatings have previously been described, often with a third component such as CrC—NiCr, on, for example, piston rings in internal combustion engines. They do not, however, have a uniform distribution of the hard phases in the range below 10 μm, but instead tend to be present in the coating as a spot landscape comprising various materials. These different materials are then present in the layer as regions having a size in the order of that of the spray powders used (which typically have 45-10 μm as indicated grain size range) so that when stressed by foreign bodies in the micron range, the coating behaves in a manner corresponding to its local composition. They are therefore not advantageous, in particular where the intrusion of foreign bodies into the tribological friction pairing must be expected. The true density of the pure fusible alloys is in the order of about 8 g/cm3, but in admixture with other spray powders slightly higher, depending on which other spray powders are mixed in.
Very high-quality coatings are those based on tungsten carbide, for example WCCo 83/17 or WC—CoCr 86/10/4. The friction behavior is advantageous due to the presence of tungstic acid or tungsten trioxide as a solid lubricant on the surface of the coating. The wear resistance is high and the layers can be produced to be pore-free, i.e., the density of the coating is in the vicinity of the true density, under suitable conditions, and have a low elongation at break. The polishability is very good because of the finely dispersed metallic matrix (Co or CoCr, alloyed with W). Layers which are under an internal compressive stress can in particular be produced, which is important for the fatigue strength of the substrate under alternating mechanical stress. Disadvantages are the very high true density of these coating materials and the resulting high geometric densities, typically up to about 14 g/cm3, the somewhat higher coefficient of friction compared to hard chromium, and the high raw materials costs for tungsten. The high geometric densities on rotating and flying components lead to an increased energy consumption due to the increased moment of inertia or the greater flying weight.
A further alternative is provided by Cr— and chromium carbide-containing alloys, in particular those based on iron and nickel, and cermet spray powders such as CrC—NiCr 75/25. Common to all these is the formation of chromium oxide (Cr2O3) on thermal spraying. This oxide is harder than metallic friction partners and scores these, but has low coefficients of friction against metallic materials. These oxide precipitates also act as predefined points of fracture of the ductile metallic matrix and reduce its elongation at break, and are thus not a priori detrimental. The self-lubricating effect due to soft oxides, which can be significant in the field of mixed friction, is absent. The true density is comparatively low and is about 7.3 g/cm3. The wear strength of these coatings is comparatively low and not satisfactory for many applications.